How do planets form?
This is a question that scientists have asked themselves for centuries. Kant (in 1755) and Laplace (in 1796) postulated the nebular hypothesis, which states that the solar system planets formed from a rotating disk of material. Over the years planet formation theory has been greatly expanded and improved with the detailed knowledge gained from observing and exploring the solar system and studying the meteoritic record. The discoveries of exo-planetary systems starting around 1995 prompted the search for new directions in planet formation. Exoplanets discovered were often much unlike the solar system with planets the size of Jupiter in very close orbits, and planet migration gained ground as one of the key elements in planet formation theory. The idea of planet migration in turn affected our ideas of how the solar system formed: the ‘grand tack’ formation model postulates that Jupiter underwent such migration as well (see PLANETPLANET, for a blog post on this topic by Sean Raymond).
In recent years, a new revolution has taken place in the world of exoplanets.
The Kepler space telescope has discovered thousands of exoplanets and planetary systems, and it has become clear that the majority of stars have planets. Again, these planetary systems are often much unlike the solar system, with super-Earths and mini-Neptunes orbiting closer to the star than Mercury does to the sun. It is clear that planet formation theory needs to be expanded with new mechanisms to explain the exoplanet population discovered by Kepler. At the same time, these new planet formation mechanisms will affect our ideas of how the solar system formed.
The workshop I attended (see below) focused on new developments in planet formation theory. Planet formation is a complex process that involves processes at multiple scales: from the initial growth of micron-sized dust grains; to the final stages of gravitational interaction between proto-planets; and many steps in between. In this blog post I will discuss a recent idea that gained a lot of attention during the workshop and that I think will play a prominent role in future planet formation theories: Pebble accretion.
Pebbles are dust grains of a size that provides them with special aerodynamic properties. They are large enough that they can move through the nebular gas and easily encounter growing planets. But pebbles are also small enough that friction with the gas surrounding growing planets makes them spiral in. The high mobility and efficient accretion makes pebbles ideal for growing planets. Pebble accretion is a likely solution to a long-standing problem in planet formation: the growth of gas giant planets like Jupiter. With gravity alone it is hard to form a ten earth-mass core fast enough to accrete the nebular gas that comprises most of Jupiter’s mass. Pebble accretion speeds up the growth process and makes sure there is still nebular gas around after Jupiter’s core has formed.
Pebble accretion may also have played a crucial role in the formation of earth and the terrestrial planets. Jupiter’s quick formation and immense gravity could have prevented many pebbles from reaching the inner solar system, preventing the formation of super-earths and mini-Neptunes that we do see around other stars. A single planet formation mechanism that explains both the solar system and exoplanetary systems is very appealing to theorist.
But how do we observationally prove that pebble accretion is indeed the mechanism that forms planets and exoplanets?
The answer to this question may lie in the chemical composition of planet atmospheres. Jupiter’s atmosphere contains a higher concentration of heavy elements than the sun, indicating that it accreted not only gas but also some solid material. This solid material could be in the form of pebbles or as larger objects called planetesimals. This size difference between the two leads to a distinct chemical composition. Pebbles easily lose their water and other volatiles when they get heated, while planetesimals are more likely to retain their water. This difference in water content will be reflected in the composition of the exoplanet atmospheres that accreted them. Using the composition of exoplanet atmospheres to trace the early steps of planet formation is an exciting prospect for our understanding of the planet formation process.
I’m excited to see where this new direction in planet formation will take us in the coming years. Will pebble accretion also become the dominant paradigm in the formation of exoplanets? What new insights into the solar system can it provide? And can we find a “smoking gun” for pebble accretion, perhaps in the composition of exoplanet atmospheres?
Footnote: I attended the workshop New Directions in Planet Formation that took place in Leiden, Netherlands from 11 Jul 2016 through 15 Jul 2016. People tweeted from the conference under the hashtag #NDIPF . The workshop was organized by Ravit Helled (Tel-Aviv, Israel), Anders Johansen (Lund, Sweden), and Chris Ormel (Amsterdam, The Netherlands). This article is inspired by talks and discussions that took place during the workshop.